The increased frequency of sexually transmitted diseases (STDs) is clinically well-established, and methods are needed to control disease progresssion and transmission. This Program Project attempts to characterize important properties of the host- pathogen interaction so that rational therapies can be devised. The Program Project includes 5 research units that represent 5 distinct infectious agents and 2 cores. Project 1 examines the biology and pathogenicity of Trichomonas vaginalis and the in vivo dynamics of trichomonad populations in terms of surface proteins and immunogenicity. Project 2 focuses on the host-treponeme interaction, with major emphasis on the adhesins of Treponema pallidum and receptors of host cells. Project 3 attempts to characterize the mechanisms by which group B streptococci adhere to epithelial cells. These 3 projects use a spectrum of techniques that include oligonucleotide and peptide sequencing and synthesis, recombinant DNA manipulations, transposon mutagenesis, fluorescene activated cell sorting and a variety of immunological and microbiological procedures. Project 4 investigates the role of cell-mediated immunity in Chlamydia trachomatis-related infections and attempts to correlate cytokine-macrophage interactions with the outcome of the disease. Project 5 utilizes anti-idiotypic reagents to modulate the immune response to hepatitis B virus antigen and to assess its vaccine potential. The last two projects require many immunological assays performed both in vitro and in vivo along with basic biological and chemical methodologies. Two cores (Clinical and Antibody) serve central support functions for many aspects of the outlined research.